Eurêka!
I am happy because I solved a problem with one of my WP plugins, which I could not get working (now-reading). The problem was not especially related to this plugin, but it took me hours to understand the cause.
Problem:
When trying to directly load a .php file in the plugin directory, it would redirect me to index.php and WP would show the 404 error page. No problem for .php files included by others.
Concrete example:
When trying to save the options of « Now Reading », the form would be sent to the following file:
/wp-content/plugins/now-reading/admin/options.php
but instead of loading this file, it would load index.php and display a 404 error.
Same thing when trying to add a book, for the following file:
/wp-content/plugins/now-reading/admin/add.php
Explanation:
So what was happening?
I tried to look at my rewrite rules, suspecting a problem with WP rewrite rules, in vain.
It appears that the solution is much simpler: my web host apache server settings are such that you cannot directly load a .php script which has write permission for group members. Same restriction for scripts located in a directory that has write permission for group members.
By comparing the now-reading directory (and subdirectories) with my other plugins’, I found that the permission was 775 instead of 755. And the file permissions were 664 instead of 644.
Solution:
I changed my directory permissions to 755 and file permissions to 644, and… it worked!
Que d’émotions 